A "trusted third party" in financial systems is commonly a backer of a proxy asset who will exchange your monetary token for the asset equivalent. As such, during the gold standard, banks served as 'trusted third parties' who would exchange dollar notes for gold, thereby providing 'backing' to the dollar.

Financial "layers" such as the lightning network serve as 'trusted third parties' in that most original sense of the phrase because they sell non-blockchain tokens in a database backed by the promise that those tokens can be exchanged for real bitcoin on the bitcoin blockchain.

A masternode on the Dash network on the other hand is part of the decentralised cryptocurrency network itself - just like any other node and facilitates the transaction of 'real' blockchain tokens in realtime, not synthetic ones in batched-time.